Permeability is the ability of a material to transmit fluid. In relation to hydrocarbon recovery, rock permeability is one of the various parameters utilized to characterize a hydrocarbon-bearing reservoir and to predict the reservoir's productivity and profitability. When a single fluid is present in a formation, an absolute permeability can be measured, for example, in millidarcy (mD). When two or more immiscible fluids are present in a formation (as is usually the case), each fluid affects the ability of the other fluids to flow. In such cases, the effective permeability is the ability to preferentially transmit a particular fluid in the presence of other fluids, and the relative permeability is the ratio of the effective permeability of the particular fluid at a certain saturation to the absolute permeability of that fluid at total saturation.
To further complicate hydrocarbon extraction from a reservoir, permeability is not typically distributed uniformly. Rather, rock formations are typically complex, heterogeneous, and anisotropic—that is, rock formations usually have varied geometry and permeability across different locations within the formation. The magnitude of permeability contrast can have significant impact on production. For example, a layer of low permeability can impede the downward movement of hydrocarbon gas, whereas a layer of high permeability can prematurely bring undesired water to a production well (referred to as water breakthrough). Wells can be completed such that the effect of the formation's heterogeneity on production is mitigated. For example, well completions can aim to evenly distribute inflow of fluids.